1. The Field of the Invention
The invention relates to earth-based satellite transmitters, and more particularly to a system and method for automatically activating a remote battery-operated earth-based satellite transmitter in response to a signal detected from an orbiting satellite.
2. The Prior Art
Satellite systems have increasingly become an important link in global telecommunication systems. Earth-orbiting satellites are now commonplace and have a wide variety of applications, including the relaying of video, audio and data information between stationary earth-based transmitting and receiving stations.
Of particular interest in connection with this invention is the ARGOS satellite system, which is administered jointly by the United States, Canada and France. The ARGOS system is composed of two polar-orbiting satellites, a network of ground tracking/command and control stations and a worldwide data dissemination system.
The primary mission of the ARGOS system is to provide scientific data regarding weather conditions, atmospheric and sea surface information, particularly in the polar regions of the earth above eighty degrees north and south latitudes, due to the fact that geostationary satellites are not able to "see" these regions of the earth because of the curvature of the earth's surface.
The two polar-orbiting satellites of the ARGOS system orbit the earth every 102 minutes at an inclination of ninety-eight degrees with respect to the earth's equator, passing near the poles with each revolution. The earth rotates or precesses approximately twenty-five degrees during each orbital period, providing full global coverage by each satellite once every twenty-four hours. The orbits of the two satellites are alternated in such a way that one satellite makes a series of passes over any given point an the earth in the early morning hours, while the other satellite provides a series of excellent passes over the same point in the late afternoon hours. This assures good, low-angle, high-contrast photos for surface feature recognition. Feature recognition is necessary in order to overlay a map outline, or "grid" the surface of the earth with latitude/longitude lines on weather photos.
A secondary mission of these two satellites of the ARGOS system is to carry a special data collection system. The data collection system consists of specialized receivers which listen continuously for transmissions from any of thousands of small transmitters (sometimes called "platform terminal transmitters" or "PTT's") which may be scattered about the surface of the earth. These small, remote transmitters are used to send a variety of meteorological, environmental and geophysical data from varying locations. Currently the ARGOS satellite system is the only system capable of full global coverage, thus making it especially significant with respect to collection of data from these remotely scattered transmitters. The data collection system on board the ARGOS system satellites checks, sorts and formats all data from the earth transmitters, and stores the information for later transmission upon command as the satellites pass over one of the ground tracking stations.
A major problem affecting the operational life of the small PTT transmitters which are scattered around the earth is the limited life of the small, portable battery packs which power them. The present technological approach to extending the transmitter life is to time each period of transmission such that the transmitter is allowed to operate only a specified number of hours a day, a number of days per week, month and so forth. During the time the transmitter is on, it actually sends a short transmission (typically less than one second in length) at regular intervals, ranging from 40-60 seconds. The length of time between transmissions is determined by whether or not the ARGOS satellite system is to receive data only from the transmitter, or whether the satellite system is actually to calculate the position of a transmitter on the earth's surface.
Since such PTT transmitters are battery operated, an operational life of a few weeks is considered to be very good. Large battery packs, such as may be placed in drifting ocean buoys when such PTT's are used to track ocean currents, have provided a maximum of from three to nine months operating life. However, PTT transmitters which require smaller battery packs, such as those used in tracking certain kinds of migratory animals, have a much shorter operational life. Since the orbital period of each ARGOS satellite is 102 minutes, and the altitude of their essentially circular orbit is approximately 820 kilometers, the maximum time of an overpass relative to a PTT transmitter unit is approximately sixteen minutes or less. Thus, using present technology the small transmitters must actually transmit continuously, even when no satellites are available for reception, which is typically a very large percentage of the time.
If a transmitter's position on the earth were accurately known, a complex computer program could predict the overpass times of each ARGOS satellite. However, such predictions remain valid for relatively short periods, typically on the order of weeks or a few months at best, due to the decay and/or forced corrections of the orbit of each spacecraft. Furthermore, in the case of PTT transmitters which are mounted on or carried by objects which move great distances, such as ships, migratory animals, weather balloons, floating buoys, etc., the time of a given satellite overpass is virtually impossible to predict because the position of the transmitter is constantly changing.
In summary, the use of PTT transmitters for tracking ocean currents, migratory animals or other such applications has been significantly limited because of the problem of limited battery life. This is due to the fact that much of the transmitted power of the PTT is wasted because of the need to transmit on a continuous or frequent basis in order to assure that such transmissions will be picked up by one of the ARGOS satellites as it passes over the transmitter.